Scientists have tested gravity across some of the largest structures in the universe and found that it behaves exactly as predicted by long-standing physical laws.
Researchers led by University of Pennsylvania used data from the Atacama Cosmology Telescope to examine how galaxy clusters move across vast cosmic distances.
Their results show that gravity weakens with distance in line with the inverse-square law first described by Isaac Newton and later embedded in Albert Einstein’s theory of general relativity.
The findings challenge alternative theories that suggest gravity changes at large scales and instead reinforce the idea that an unseen component, dark matter, is shaping cosmic motion.
Gravity holds at scale
“Astrophysics has been plagued by a massive discrepancy in the cosmic ledger,” said Patricio A. Gallardo.
“When we look at how stars orbit within galaxies or how galaxies move within galaxy clusters, some appear to be traveling way too fast for the amount of visible matter they contain.”
To test whether gravity itself might be responsible, the researchers analyzed subtle distortions in the cosmic microwave background as it passes through massive galaxy clusters.
These distortions, caused by the motion of hot gas around clusters, allowed the team to measure how quickly clusters are moving toward each other across distances spanning hundreds of millions of light-years.
The results closely matched predictions from classical and relativistic physics, showing no evidence that gravity weakens differently than expected at these scales.
“It is remarkable that the law of the inverse of the squares—proposed by Newton in the 17th century and then incorporated by Einstein’s theory of general relativity—is still holding its ground in the 21st century,” said Gallardo.
Dark matter case strengthens
The study addresses a long-standing puzzle in cosmology. Observations have consistently shown that stars at the edges of galaxies and galaxies within clusters move faster than visible matter alone can explain.
“That is the central puzzle,” Gallardo explained.
“Either gravity behaves differently on very large scales, or the universe contains additional matter that we cannot directly see.”
Because the new measurements confirm that gravity behaves as expected, the results strengthen the case for dark matter as the missing component.
“This study strengthens the evidence that the universe contains a component of dark matter,” said Gallardo. “But we still do not know what that component is made of.”
The work also places constraints on theories such as Modified Newtonian Dynamics, which attempt to explain cosmic motion by altering the laws of gravity.
By extending tests of gravity to distances far beyond the scale of individual galaxies, the research provides one of the most comprehensive validations of standard cosmological models to date.
Future observations using more detailed maps of the cosmic microwave background and larger galaxy surveys could further refine these measurements and test gravity with even greater precision.
“With so many unanswered questions, gravity remains one of the most fascinating areas of research. It’s a naturally attractive field,” Gallardo said.